Forged or Stamped Average or Small Size Mechanical Part

ABSTRACT

The inventive average or small size mechanical steel part is produced by hot forging or cold stamping i.e. by plastic processing of a long ferrous semi-product which is obtainable by continuous casting and hot rolling in the austenitic phase and, afterwards is shaped by plastic deformation and heat treated in order to obtain a metallographic structure substentially containing an acicular ferrite at least in mechanical toughness and fatigue stress areas. The composition of said steel, apart from iron and inevitable residual impurities resulting from steel production, corresponds at least to the following analysis: 0.2-0.5% C, 0.5-2.0% Mn, 0.05-0.5% V, 0.6 1.5% Si, 0.05 1.0% Cr, 0.01-0.5% Mo, 0.02-0.10 S, preferably from 0.01 to 0.02% Ti and/or up to 0.20% Al, and from 5 to 30 ppm of Ca.

The invention relates to mechanical parts of medium or small size madeof medium carbon micro-alloyed steel, such as wheel hubs, connectingrods or swivels for an automobile, or other similar mechanical partsobtained through hot or cold plastic deformation of a long siderurgicalsemiproduct and for which there are sought, above all, properties ofresistance to fatigue and of tenacity. By medium or small size, there isunderstood here parts the diameter of which does not exceedapproximately 80 mm.

In order to produce such parts, it is known to make use of steelsspecially alloyed to obtain a metallographic structure of a bainitic oressentially bainitic type. By “essentially,” there usually must beunderstood 80% and more by volume of the bainitic structure at the placeon the part where this structure is sought.

Their manufacture in fact requires being able to withstand significantmodifications in form without breaking or cracking, while in the endaffording a good resistance to fragile breaking (tenacity) and fatiguein view of the cycles of repetitive stresses to which the parts aresubjected in use, as well as to impacts (high resilience). Furthermore,these steels must afford good machinability characteristics, in order topermit a precise final dimensioning by machining of the part ready foruse required in a number of applications.

The manufacturing process usually can comprise an operation of cold(press or forge) or hot (forge) plastic deformation, the choice of thehot or cold method often being made according to the final size of theparts. In all cases, this operation will be performed on pieces of steelcut up into bars deriving from long, continuously cast hot-rolledsiderurgical semiproducts. When the plastic deformation is performed“hot,” the pieces of steel are reheated beforehand to a temperature ofapproximately 1000 to 1200° C., then hot-formed in the forge. The partsobtained then are cooled and treated thermally by hardening andtempering. When the plastic deformation is performed “cold,” the piecesare cold-formed in the press, possibly after having undergone aglobularization annealing. The parts obtained then are treated thermallyby hardening and tempering.

It is recalled that in service, these parts ordinarily are subjected tovariable, even cyclic, mechanical stressing, which generates asignificant fatigue effect. Steel fatigue is expressed by the occurrenceof microfissures that propagate until breaking, even if the stress islower than the tensile strength or the limit of elasticity of the metalthat constitutes the part. Nowadays it is estimated that fatigue isresponsible for nearly 90% of the breaking of mechanical parts inservice. Likewise, the impacts that a mechanical part may undergo inservice bring about the occurrence of microfissures that can cause thepart to break prematurely if particular attention is not given to theresilience properties of the metal that constitutes it.

Now, the bainitic structure of the steel ordinarily appears in the formof parallel laths that consequently present few obstacles to thepropagation of microfissures. This structure, although sought for itsproperties of mechanical resistance and ductility, does not necessarilyafford a satisfactory tenacity or resistance to fatigue.

It is known, for example through document EP 0 787 812, to improve thefatigue resistance of forged parts by virtue of the presence of residualaustenite within the bainite, obtained by means of an appropriatecontrolled cooling combined with the choice of a grade of steel thecomposition of which was specially enriched with silicon.

The purposes of the invention is to contribute another solution to theimprovement of the fatigue resistance and tenacity of forged or pressedmechanical parts that preserves their high mechanical characteristics,for example of resistance, ductility and resilience.

To this end, the invention has as its purpose a mechanical part made ofsteel deriving from the hot forge or the cold press, of medium or smallsize, resulting from the plastic deformation of a long siderurgicalsemiproduct, characterized in that the steel of which it is composed hasa composition which, besides iron and the inevitable residual impuritiesresulting from the processing of steel, corresponds at least to thefollowing analysis, given in weight percentages:

-   -   0.2≦C≦0.5    -   0.5≦Mn≦2.0    -   0.05≦V≦0.5    -   0.6≦Si≦1.5    -   0.05≦Cr≦1.0    -   0.01≦Mo≦0.5    -   0.02≦S≦0.10    -   and possibly up to 50 ppm boron        and in that the said part is obtained from a long semiproduct        deriving from continuous casting and hot-rolled in the        austenitic area, then formed by plastic deformation and treated        thermally to obtain a metallographic structure containing        essentially acicular ferrite, at least in the zones of        mechanical stressing in tenacity and fatigue.

By “essentially” there is understood here at least 50% and preferably60%, or even advantageously 80% and more of acicular ferrite by volume.

The invention further has as its purpose a steel for the manufacture ofa mechanical part by plastic deformation characterized in that, besidesthe inevitable residual impurities resulting from the processing ofsteel, its chemical composition includes at least, expressed in weightcontent:

-   -   0.2≦C≦0.5    -   0.5≦Mn≦2.0    -   0.05≦V≦0.5    -   0.6≦Si≦1.5    -   0.05≦Cr≦1.0    -   0.01≦Mo≦0.5    -   0.02≦S≦0.10    -   and possibly up to 50 ppm of B and in that the metallographic        microstructure that it will have, once the said part is        implemented, is composed essentially of acicular ferrite at        least in the zones of the part subjected to mechanical stressing        in tenacity and fatigue.

With regard to both the mechanical part and the grade of steel definedhereinabove, in order to facilitate the obtaining of acicular ferrite,the steel furthermore includes preferably 5 to 30 ppm of Ca, and/or 0.01to 0.02% Ti, with possibly up to 0.2% Al.

The invention further has as its purpose a process for manufacture ofsuch a mechanical part made of steel characterized in that, with thegoal of obtaining acicular ferrite at least locally on the said piece,it comprises the following stages:

there is supplied a continuous-casting billet made of steel of acomposition in accordance with the analysis given hereinabove, that ishot-rolled at a temperature in excess of 1000° C. into a bar or wirebefore being cooled to room temperature after rolling;

the wire being subjected to a controlled cooling prior to its formationinto a ring for obtaining a metallographic structure composedessentially of acicular ferrite, which wire then is cut into pieces andcold-pressed into a finished part ready for use;

the bar itself being cooled naturally in the rolling heat prior to itscutting into pieces which then are hot-forged into a rough shape of apart which is cooled by controlled cooling for obtaining a structureessentially composed of acicular ferrite at least in the stressed zonesof the part, which rough shape then is machined, as need be, to thedesired dimensions to make it into a finished part ready for use.

In a variation, the controlled cooling is a natural cooling to roomtemperature. In practice, in fact, it happens that the forged parts arestored immediately in bulk in buckets, on top of each other. The partslocated on the top of the pile are going to cool more rapidly than thoselocated underneath. A controlled cooling of each piece, therefore, isnot sought at this stage, since they usually then are going to betreated thermally anyway.

On the other hand, in the process according to the invention, the partscertainly may cool naturally (that is, without blowing of air), but thiscooling nonetheless must be controlled in order to ensure the formationof acicular ferrite. This control of the cooling may be accomplished,for example, by depositing the parts one by one, apart from each other,directly after the forge operation, on a conveyor belt that transportsthem to the receiving area of the works with a view to their storageprior to shipment.

According to a preferred variation of the invention, however, thecontrolled cooling is a forced cooling, for example with blown air,ensuring a surface cooling speed of approximately 0.5 to 15° C./s.

It is recalled that vocabulary practices in the siderurgical tradeprovide that rolled products with diameters ranging up to approximately30 mm in diameter (which frequently are packaged in the form of rings)are referred to as “wire,” and those rolled products starting from 18 mmin diameter and which are delivered straight after cutting lengthwise atthe rolls outlet are referred to as “bars.”

Finally, the invention has as its purpose a long, medium carbonsiderurgical semiproduct, intended to be transformed by forge or coldpress into a mechanical part with high characteristics, of small size ormedium size, characterized in that the steel that constitutes itcorresponds to the following analysis, given in weight percentages:

-   -   0.2≦C≦0.5    -   0.5≦Mn≦2.0    -   0.05≦V≦0.5    -   0.6≦Si≦1.5    -   0.05≦Cr≦1.0    -   0.01≦Mo≦0.5    -   0.02≦S≦0.10    -   and possibly up to 50 ppm of boron        and in that the metallographic microstructure that it will have        after transformation will be composed essentially of acicular        ferrite at least in the zones of the part subjected to        mechanical stressing in tenacity and fatigue.

As undoubtedly will have been understood, the invention in fact consistsin proposing the manufacture of a tough, resilient mechanical partendowed with a microstructure essentially composed of acicular ferriteat least in the zones of the part mechanically stressed in fatigue, froma medium carbon steel combined, in the analysis brackets given in theseelements, with manganese (itself also gammagenic) for resistance tobreaking, and micro-alloyed with vanadium supported by sulfur in orderto promote the development of acicular ferrite and combined, firstly,with molybdenum in order to improve resilience and to harden the ferriteeven more than the vanadium alone, secondly, with chromium in order tofacilitate the effectiveness of the controlled cooling at the time ofthe transformation operation, and thirdly, with silicon, itselfalphagenic, to increase resilience, but also to favor the precipitationat the grain joints of a ferrite which will prevent the bainite frominvading everything and thus will allow the acicular ferrite to appearin order to take its rightful place.

It should be recalled here that acicular ferrite is a metallographicconstituent known in siderurgy. It already is used, for example, asshown in EP-A No. 0288054, to facilitate the process for manufacture offine-grained sheets for low-temperature use (offshore, etc.) byeliminating the intermediate reheating state between casting andhot-rolling.

Likewise, as shown in U.S. Pat. No. 6,669,789, it is known to make use,aside from the customary polygonal ferrite-perlite, of an acicularferrite structure (that forms on the carbides) for the manufacture oftitanium steel sheet with high resistance and ample elongation in orderto limit the austenitic grain size starting from hot-rolled thin slabs.

The invention will be well understood and other aspects and advantageswill emerge more clearly in view of the detailed description thatfollows, given by way of an embodiment example.

There are produced by continuous casting in the steelworks longsemiproducts (billets or blooms) deriving from a steel having, besides,iron, the following composition by weight content in relation to theiron:

From 0.2 to 0.5% carbon. At these contents, the carbon makes it possibleto obtain good mechanical resistance characteristics. In particular, therequired resilience is ensured by the 0.2% minimums. On the other hand,the content thereof should not be too high (approximately 0.5% maximum),in order not to favor the formation of bainite instead of the soughtacicular ferrite.

From 0.5 to 2.0% manganese. Manganese ordinarily is used here in orderto increase the temperability of the steel with the aforementionedcarbon contents. However, the content thereof preferably is less than2.0% in order to avoid its segregation which would impair thehomogeneity of the structure.

From 0.05 to 0.5% vanadium. Vanadium favors the development of acicularferrite, as already stated, by making it possible to increase the sizeof the bainitic areas and by shifting them to the high temperatures. Italso reduces the area of occurrence of perlite ferrite.

From 0.02 to 0.10% sulfur. Sulfur not only improves the machinability ofthe parts, but performs a function mainly sought here in the mechanismof nucleation of the acicular ferrite. It has been discovered, in fact,that it is the sulfurs, and not the carbides as in the case of thedocument U.S. Pat. No. 6,669,789 mentioned above, which actuallyconstitute essential anchoring points on which the grains of acicularferrite, the development of which is going to be promoted by thevanadium, alloyed with silicon, are going to form.

From 0.6 to 1.5% silicon. Silicon usually serves to deoxidize the steel.Its content here, however, should remain below 1.5% in order not toweaken the steel. Here it performs an essential function in thecontrolled growth of the bainitic area in which the acicular ferrite isformed by precipitating the primary ferrite at the grain joints, asalready indicated, and thus allowing the vanadium to promote thedevelopment of acicular ferrite.

From 0.05 to 1.0% chromium. The chromium makes it possible to adjust thetemperability of the grade and thus to follow the increase in size ofthe parts to be produced. It also acts with the silicon in order toincrease the range of presence of the acicular ferrite.

From 0.01 to 0.5% molybdenum. Molybdenum contributes to the obtaining ofthe final structure through an adjustment of the temperability of thegrade. In fact, if the content of tempering elements is too low, aferrito-perlitic structure will be obtained, and conversely, anexcessively tempering grade may lead to the obtaining of martensite orresidual austenite.

Optionally, but highly recommended in practice, from 0.01 to 0.02%titanium in order to protect the elements from nitrogen and inparticular to keep free vanadium in sufficient quantity, for otherwiseit might form precipitated nitrides too readily.

Likewise optionally, but ordinarily widely used in practice, from 5 to30 ppm of calcium in order to improve the castability of the steel andits implementation. It facilitates the obtaining of oxide inclusionswhich may enter into the mechanism of nucleation of the acicularferrite.

Possibly up to 50 ppm of boron that will act in synergy with themolybdenum to broaden the bainitic area in which the acicular ferrite isformed.

Possibly up to 0.2% aluminum for control of the austenitic grain size,but it also will perform a function in the preservation of vanadium.

This optimized composition makes it possible for the steel to have,following a controlled cooling, a structure essentially composed ofacicular ferrite. By essentially there will be understood an acicularferrite content of more than 50% and preferably more than 60%, andadvantageously approximately 80% or even more. Such a metallographicstructure makes it possible for the steel to have good mechanicalcharacteristics of resistance, hardness and ductility, but also anenhanced resistance to impacts and to fatigue effect.

As is going to be seen, the acicular ferrite is obtained before or afterforming of the part, but in any case by means of a controlled cooling ofthe steel.

In the first case, deformation is performed cold on a steel alreadyhaving a structure essentially composed of acicular ferrite. There isprovided a long semiproduct composed of a steel with an analysisaccording to the invention, which is hot-rolled as need be after areheating above 1100° C., in accordance with customary hot-rollingpractice, until obtaining of a rolled wire 10 mm in diameter, forexample. The removal temperature of the wire is on the order of 900 to950° C. The rolled wire obtained is cooled with blown air in the rolling“heat” itself in the customary manner (“Steelmor” process, for example).If its diameter so permits, the wire also may be cooled naturally to theambient atmosphere.

The rolled wire is delivered in ring form to the transformer which isgoing to cut it into pieces of required length and subject them to acold press for obtaining of the desired parts. The final mechanicalcharacteristics are obtained naturally by the cold-drawing resultingfrom forming.

In the second case, plastic deformation is performed “hot” and themetallographic structure is obtained directly on the rough forge shapes.There is provided a long semiproduct composed of a steel with ananalysis according to the invention, which is hot-rolled until giving ita diameter of 35 mm, for example. After possible cooling, which does notneed to be controlled at this level, the bar is positioned lengthwiseand delivered to the smith customer.

The bars then are cut into pieces. Each piece is brought to atemperature of at least 1100° C. by means of an induction furnace. Thisheating also can be performed more classically, but the heatingconditions (heating time, speed, etc. . . . ) then must be optimized inorder to obtain a homogeneous austenitic structure having a grain sizefavorable to the formation of acicular ferrite. The austenitic grainsize then is estimated at 80 μm. The pieces are subjected to a hotplastic deformation operation. Forging is concluded at a temperature inexcess of 1100° C. The rough shapes of parts obtained in this mannerthen undergo a forced cooling to room temperature at a cooling speedranging from approximately 0.5 to 15° C./s, depending on the diameter ofthe part and the optimization of the steel composition. The part alsomay be cooled in a natural but controlled manner by placing the roughshapes at the forge outlet one by one on a conveyor belt, for example.The part then is machined to conform to the final intended dimensions.Instead of machining, the part possibly may be subjected to a secondplastic deformation. This additional operation may be carried out coldwithout running the risk of cracking the part because of the ductilenature imparted to the steel by the microstructure. It is not necessaryto implement a thermal hardening and tempering in order to obtain theintended mechanical characteristics.

The grade of steel according to the invention makes it possible toobtain a part with metallographic structure essentially composed ofacicular ferrite. It has the mechanical characteristics of resistance tobreaking and hardness required for its usage properties, and meets therequirements for machinability. In addition, it has an increasedtenacity by virtue of its very structure, in which the entanglement ofthe laths serves as a barrier to the occurrence and the propagation ofcracks. This increased tenacity in fact enables it, as a result, also tohave a better resistance to impacts and a better resistance to fatigue.Furthermore, is also makes possible a second cold forming by press, forexample. The obtaining of acicular ferrite also make it possible toincrease the mechanical resistance of the grade through the ampledispersal density of its laths.

Tests were conducted in the laboratories of the producer of semiproductsfor a forge deriving from continuous casting. A wheel hub was forgedthere from a steel according to the invention, the chemical compositionof which, besides iron and the impurities resulting from processing,correspond to the following analysis: % C % Mn % V % Si % Cr % Mo ppm B% S ppm Ca % Ti % Al 0.31 1.33 0.12 1.18 0.28 0.03 20 0.04 11 0.015 0.02

Prior to forging, this piece was heated to 1200° C. by induction. Theend temperature of forging is 1100° C. After forging, the rough shape iscooled at a speed of 2° C./s directly in the heat. No other thermaltreatment is applied.

The structure obtained on this test hub is 80% acicular ferrite; it alsohas the following mechanical characteristics: Rm (MPa) Rp_(0.2) (MPa)Hardness (HV) A (%) Z (%) 1150 800 300 11 25

It is recalled that:

-   Rm represents the resistance to breaking corresponding to the    maximal force before breaking with reference to the initial section    of the wire.-   Rp_(0.2) represents the conventional limit of elasticity    corresponding to the force with reference to the initial section of    the wire producing a plastic elongation of 0.2%.-   A represents the breaking elongation.-   Z represents the area contraction corresponding to the reduction of    the wire section after breaking.

It goes without saying that the invention could not be limited to theexample that has just been described, a wheel hub, but that it extendsto numerous variations or equivalents, in type of parts and in size anddimension, insofar as the definition thereof given in the attachedclaims is observed.

1. A mechanical part made of steel derived from the hot forging or thecold pressing thereof, of medium or small size, and resulting fromplastic transformation of a long siderurgical semiproduct, which thesteel of which it is composed has a composition that, besides iron andthe inevitable residual impurities resulting from processing of thesteel, corresponds at least to the following analysis, given in weightpercentages: 0.2≦C≦0.5, 0.5≦Mn≦2.0, 0.05≦V≦0.5, 0.6≦Si≦1.5, 0.05≦Cr≦1.0,0.01≦Mo≦0.5, and 0.02≦S≦0.10, and optionally up to 50 ppm of boron,wherein the said part is obtained from a long semiproduct derived fromcontinuous casting and hot-rolling in the austenitic area, then formedby plastic deformation and treated thermally in order to obtain ametallographic structure containing essentially acicular ferrite atleast in the zones of mechanical stressing in tenacity and fatigue. 2.The mechanical part according to claim 1, wherein the steel furthercomprises from 0.01 to 0.02% titanium and/or up to 0.20% aluminum. 3.The mechanical part according to claim 1, wherein the steel furthercomprises between 5 and 30 ppm of calcium.
 4. A steel for themanufacture of a mechanical part by plastic deformation, wherein,besides the inevitable residual impurities resulting from processing ofthe steel, its chemical composition comprises at least, expressed inweight content: 0.2≦C≦0.5, 0.5≦Mn≦2.0, 0.05≦V≦0.5, 0.6≦Si≦1.5,0.05≦Cr≦10, 0.01≦Mo≦0.5, and 0.02≦S≦0.10, and optionally up to 50 ppm ofB, wherein the metallographic microstructure that the steel will have,once the part is implemented, is essentially composed of acicularferrite at least in the zones of the part subjected to mechanicalstressing in tenacity and fatigue.
 5. The steel according to claim 4,wherein, in order to protect the vanadium, the steel further comprisesfrom 0.01 to 0.02% titanium and/or up to 0.20% aluminum.
 6. The steelaccording to claim 4, further comprising between 5 and 30 ppm ofcalcium.
 7. A process for the manufacture of a mechanical part made ofsteel, wherein, for the purpose of obtaining acicular ferrite at leastlocally on the part, the process comprises the following stages:providing a continuous casting billet made of steel with a compositionaccording to claim 4, which is hot-rolled at a temperature in excess of1000° C. into a bar or wire before being cooled to room temperatureafter rolling; subjecting the wire to a controlled cooling prior toformation into rings to obtain a metallographic structure composedessentially of acicular ferrite, which wire then is cut into pieces andcold-pressed into a finished part ready for use; and cooling the barnaturally in the rolling heat prior to cutting the bar into pieces whichthen are hot-forged into a rough shape of a part that is cooled bycontrolled cooling to obtain a structure essentially composed ofacicular ferrite at least in the stressed zones of the part, which roughshape then is machined, as need be, to the desired dimensions to make itinto a finished part ready for use.
 8. The process according to claim 7,wherein the controlled cooling is a natural cooling to room temperature.9. The process according to claim 7, wherein the controlled cooling is aforced cooling ensuring a surface cooling speed of approximately 0.5 to15° C./s.
 10. A long, medium carbon siderurgical semiproduct, intendedto be transformed by forge or by press into a mechanical part with highcharacteristics, of small size or of medium size, wherein, in order thatthe part may have a metallographic microstructure essentially composedof acicular ferrite at least in the zones of the part subjected tomechanical stressing in tenacity and fatigue, the steel that constitutesthe part corresponds at least to the following analysis, given in weightpercentages: 0.2≦C≦0.5, 0.5≦Mn≦2.0, 0.05≦V≦0.5, 0.6≦Si≦1.5, 0.05≦Cr≦1.0,0.01≦Mo≦0.5. and 0.02≦S≦0.10, and optionally up to 50 ppm of boron,wherein the metallographic microstructure that it will have aftertransformation will be essentially composed of acicular ferrite at leastin the zones of the part subjected to mechanical stressing in tenacityand fatigue.